In linear systems that are driven by a periodic input, the existence of noise
generally reduces the regularity of the output (e.g. the coherence between the
input and the output). This is not necessarily the case in nonlinear systems:
Excitable or bistable nonlinear systems with a threshold and with noise,
which are driven by a sinusoidal sub-threshold input, can show maximum
coherence between the input and the output for an intermediate noise level,
for which the leading output frequency of the system is close to the input
frequency. This phenomenon is called stochastic resonance (SR)
[1]}, [2]}. SR has been suggested to explain the
characteristic timing of DO events [3]}, i.e. the apparent
tendency of the events to occur with a spacing of about 1470 years or integer
multiples thereof. It has further been demonstrated that DO events in the
model CLIMBER-2 can be subject to SR [4]}.
